I just wanted to share this as my mum discovered it and it has saved her a fortune!
If your income is less than £15k Welsh Water will cap your water charges to £190 for the tax year in which you apply and for subsequent years. So, if you apply now, you would get the cap for 17/18 (and get a refund if applicable) then 18/19 going forward. Even if your income then goes up, you would still get the discount until the next tax year, as I understand it.
www.dwrcymru.com/en/My-Account/Helpu.aspx
Benefits such as DLA/PIP aren't counted when calculating household income.
Hope this helps someone!